Conventionally, a fuel injector supplying and injecting a fuel to an internal combustion engine includes a nozzle which injects the fuel and an actuator which drives to open or close the nozzle. The nozzle (fuel injection nozzle) used in the fuel injector includes a nozzle body which is a cylindrical shape and a needle which is received in the nozzle body and is slidable relative to an inner periphery of the nozzle body. In the nozzle, the inner periphery of the nozzle body includes a seat surface, and an injection port is placed at a tip end side of the seat surface in an axial direction of the nozzle body. The needle includes a seat portion which is removed from or seated on the seat surface to start or stop a fuel injection using the injection port.
However, in a fuel injector in which a fuel with high pressure is directly injected into a cylinder of a diesel engine, a request of an emission reduction is high. Therefore, as an aspect of the emission reduction, it is considered that a penetration of a spray of the fuel is maintained to reduce a smoke generation even though a lifting amount of the needle is small and an injection quantity is significantly small.
According to JP-H08-144895A, since a center diameter of a needle is greater than an inner diameter of a sack portion, a spray can be properly generated and a non-combustion gas can be reduced. However, when a lifting amount is small before a throttle portion of a flow of a fuel becomes an injection port in a nozzle, a flow-passage area is sharply enlarged toward downstream from a position where a diameter of the needle is the center diameter. Therefore, when the lifting amount is small, a cavitation is generated in the sack chamber such that a flow coefficient is deteriorated, and the penetration of the spray is decreased such that a smoke is readily generated.